The present invention relates to an exhaust gas recirculation system for an internal combustion engine.
It is known to use exhaust gas re-circulation (EGR) to reduce engine exhaust emissions. The exhaust gas that is to be re-circulated must be cooled to keep intake manifold temperatures low. Currently, the heat is removed from the EGR loop and absorbed in the water/glycol circuit that cools the engine. This works, but it is not an efficient method of removing heat because the hot exhaust gas transfers its heat to cooler liquid which then transfers its heat to the ambient air. Since heat transfer is driven by a difference between the two mediums, it is always best to keep this difference as large as possible.
In a low-pressure loop exhaust gas recirculation (EGR) system, the exhaust is removed downstream of the turbine, and re-inserted after the air cleaner and before the compressor. The combined combustion air and exhaust gas is then compressed and sent to the charge air cooler where it is cooled prior to entering the intake manifold of the engine.
During cool weather, water may condense out of the combined charge air. This condensed water is very corrosive because the exhaust gas contains both sulphuric and nitric acids. The corrosive condensate will damage the charge air cooler which is usually made of copper alloy or aluminum for efficiency reasons. An EGR system which reduces or eliminates such corrosive condensation is desired.
U.S. Pat. No. 6,216,458 shows an EGR system with a gas to air EGR cooler and a valve in an air line between the EGR cooler and exhaust gas outlet. However, this patent describes embodiments which require pressurized cooling air from the intake air compressor, and a mass air flow sensor. In one embodiment fresh ambient air is communicated to the EGR cooler, but the fresh air to be transported to the EGR cooler is “ram air” or air which is drawn through the radiator with a fan as the vehicle travels at a given speed. This embodiment includes both a temperature sensor and a pressure responsive EGR valve.